This invention relates in general to a semiconductor device structure and more particularly to a semiconductor die mounted and bonded to a plateless copper alloy package.
A semiconductor device typically includes a semiconductor die and a package or housing for the die. The package includes a die mount portion and a lead portion. The die, including diffused regions, junctions, and the like, is metallized on both its top and bottom surfaces. A solder is used to bond the bottom metallization to the die mount portion of the package. Wire leads bond to and interconnect the top metallization and the package lead portion. The die and interconnecting leads are then enclosed within the package. The enclosure comprises either plastic encapsulation molded about the die and part of the die mount and lead portions or a metal cover welded to the mounting portion and extending over the die.
The die mount portion of the package provides mechanical support, electrical contact, and functions as a heat sink. The die is usually attached to the die mount portion using soft solder, hard solder, or conductive epoxy. Each of these mounting materials has characteristic advantages and disadvantages. The soft solders, for example, which are lead-based or tin-based alloys, are inexpensive, but are susceptible to thermal fatigue and higher electrical and thermal resistance than the hard solders. Hard solders are gold-based alloys and provide highly reliable bonds having good thermal, electrical, and mechanical properties, but the gold-based hard solders are very expensive. Conductive epoxies, like the soft solders, are less expensive but provide less desirable bonds than do the hard solders and often contain precious metal fillers.
The back of the semiconductor die is metallized to provide a bondable surface. When solder is used for bonding the back metallization must be compatible with the solder chosen. Likewise, the solder must be compatible with the metal of the die mount portion. That is, the back metal, solder, and die mount metal must all be metallurgically compatible. Metallurgically compatible means the solder is capable of wetting and forming a strong bond to the metallic surfaces it contacts but forms no undesirable intermetallics at the bond. In contrast, gold and tin can form a brittle intermetallic which is fracture prone and would therefore provide a low reliability bond.
The requirement of compatibility has typically led to the use of clad or plated die mount regions. The underlying material for the die mount region is selected for its thermal, electrical and mechanical properties. The cladding or plating provides the required metallurgical compatibility. Depending on the solder system chosen, the die mount region is overcoated with a thin layer of material, usually either nickel or gold. This adds the expense of the plating or cladding operation as well as the cost of the material itself to the total cost of the device.
The front surface of a semiconductor die is metallized with a patterned metal layer which allows electrical contact to, for example in the case of a bipolar transistor, the base and emitter electrodes of the transistor. This patterned metallization is connected to the lead portion of the semiconductor package by fine wires (typically in the range of 25-500 micrometers in diameter) which are bonded between the surface metallization and the package leads. The wires are usually aluminum or gold, the top surface metallization is aluminum or a number of alternative multi-layer metal systems, and the package leads are plated with nickel or gold. The lead plating or cladding is necessary for metallurgical compatibility between the aluminum or gold wires and the rigid material of the package leads. Here there is no requirement for solder wetting, but metallurgical compatibility again requires that no undesirable intermetallics form at the bond. Again, the plating or cladding of the package leads is expensive and further expense is added by the gold wires, where used.
The total semiconductor device, assembled as described above, is beset by a number of shortcomings. Assembling the semiconductor device requires the selection from among various metallurgical alternatives and requires tradeoffs with regard to such variables as cost, thermal and electrical properties, and reliability. Reliability, for example, may be compromised by the formation of aluminum-gold intermetallics. Because of these shortcomings and required tradeoffs, a need existed for an improved semiconductor device assembly.
It is therefore an object of this invention to provide an improved semiconductor device assembled on a plateless copper alloy package member.
It is a further object of this invention to provide an improved metallurgically compatible back-surface metallization, metallic solder, and unplated die mount package member.
It is a still further object of this invention to provide a front-surface metallization and lead bonding means compatible with an unplated copper alloy package lead portion.
It is a still further object of this invention to provide an improved device metallization forming no intermetallics.
It is another object of this invention to provide a semiconductor die metallurgically bonded to an unplated copper alloy die mount package member and lead bonded to an unplated copper alloy lead portion package member.
It is still another object of this invention to provide a housed semiconductor device mounted on an unplated mounting member package portion and having improved reliability properties.